1. Field of the Invention
The present invention relates to a color cathode ray tube (CRT), and more particularly, to an in-line electron gun having improved electrodes for forming a large electronic lens.
2. Description of the Related Art
In a general electron gun for a color cathode ray tube, spherical aberration and focusing characteristics are greatly dependent on a main lens. Thus, in order to obtain good focusing characteristics, the diameter of the main lens should be increased.
However, in an in-line electron gun, since three electron beam apertures are formed at at least two electrodes constituting an electronic lens it is impossible to make the diameter of an electron beam aperture larger than a distance between the centers of two adjacent electron beam apertures, which will be called an xe2x80x9ceccentric distancexe2x80x9d hereinafter.
An electron gun for improving spherical aberration in a conventional main lens is disclosed in U.S. Pat. No. 4,370,592, and is shown in FIG. 1.
As shown in FIG. 1, burring portions 1b and 2b are located at edges of an electron outlet surface 1a of a focusing electrode 1 and an electron inlet surface 1a of a final accelerating electrode 2, and large electron beam apertures 1H and 2H having a predetermined depth are located in the center. Also, small electron beam apertures 1Hxe2x80x2 and 2Hxe2x80x2 through which R, G and B electron beams pass independently, are located in the large electron beam apertures 1H and 2H.
When electron beams pass through a main lens formed by the focusing electrode 1 and the final accelerating electrode 2, since vertical and horizontal focusing field components acting on the electron beams passing through the central small electron beam aperture and the side small electron beam apertures are different due to horizontal elongation of the large electron beam apertures 1H and 2H, it is not possible to form symmetricral electron beam spots landing on a fluorescent surface. In other words, as shown in FIG. 2, side electron beams RB and BB having passed through the large electron beam apertures 1H and 2H of the focusing electrode 1 or the final accelerating electrode 2, are close to the burring portions 1b and 2b across which a low voltage or a high voltage is horizontally applied, and the central electron beam GB is relatively far from the burring portions 1b and 2b. Thus, the side electron beams RB and BB are relatively strongly focused and the central electron beam GB is weakly focused.
Also, since the distances between the side electron beams RB and BB and the burring portions 1b and 2b are different according to direction, the horizontal focusing power and vertical focusing power for the electron beams are different. Further, the vertical distances between the central electron beam GB and the burring portions 1b and 2b are shorter than the horizontal distances, so the electron beam is subjected to relatively strong vertical focusing power. Also, the central electron beam GB is subjected to divergent power in a diagonal direction of the large electron beam apertures 1H and 2H. Thus, the cross sections of the side electron beams RB and BB having passed through the main lens are substantially triangular and the cross section of the central electron beam GB is radially projected, thereby preventing the attainment of uniform cross sections of electron beams throughout the entire fluorescent plane.
In particular, the sizes of the small electron beam apertures 1Hxe2x80x2 and 2Hxe2x80x2 are limited by the diameter of a neck portion of a CRT. This sets a limit on increasing the eccentric distance between the small electron beam apertures 1Hxe2x80x2 and 2Hxe2x80x2. Further, since the recent trend is toward reduction of the diameter of the neck portion in order to reduce deflection power, the distance between the small electron beam apertures 1Hxe2x80x2 and 2Hxe2x80x2 is reduced, thereby increasing spherical aberration and degrading focusing characteristics.
Technologies for solving the above-described problems are disclosed in U.S. Pat. Nos. 5,481,560, 4,599,534 and 4,412,149, in which independent small electron beam apertures are vertically elongated in inner electrodes installed inside an outer electrode for an electron beam, or side electron beam apertures are formed using the inner electrode and the inner circumferences of the outer electrode.
In the above-described electrode system for forming a main lens, electron beams passing through side electron beam apertures are vertically elongated to reduce a focus voltage difference due to the large diameter of the outer electrode. However, the electron beam landing on a phosphor layer is severely distorted due to the high current density of upper and lower parts of the vertically elongated electron beam.
Another arrangement of electrodes of an electron gun for solving the above-described problems is disclosed in U.S. Pat. No. 5,414,323. As shown in FIG. 3, this arrangement of electrodes of an electron gun includes an electrode plate member 12 in the center of an outer electrode 11 having a large electron beam apertures. A vertically elongated small electron beam aperture 13 is located in the center of the electrode plate member 16. Side edge portions of the electrode plate member 12 are recessed in a half-elliptical shape to form side electron beam apertures 14 and 15.
Since the central small electron beam aperture is vertically elongated, the astigmatism generated by the large electron beam aperture can be offset. However, according to this electrode arrangement, 8-pole coma aberration of the central electron beam aperture and 8-pole coma aberration of side electron beam apertures cannot easily be corrected.
Another arrangement of large electrodes is disclosed in U.S. Pat. No. 4,626,738. As shown in FIG. 4, this arrangement of electrodes includes an outer electrode 21 having a large electron beam aperture, and an inner electrode 22 installed inside the outer electrode 21 and having polygonal small electron beam apertures 22R, 22G and 22B. Here, although the aberration generated by the large electron beam aperture can be corrected by the polygonal small electron beam apertures 22R, 22G and 22B, it is not easy to fabricate the small electron beam apertures 22R, 22G and 22B.
To solve the above problems, it is an object of the present invention to provide an electrode system of an electron gun for a color cathode ray tube, which can easily correct aberration of an electronic lens formed by a large electron beam aperture, and can improve focusing characteristics.
It is another object of the present invention to provide an electron gun for a color cathode ray tube, which can reduce astigmatism by compensating for distortion of an electron beam due to a difference in the voltage applied across the space between each of three apertures disposed in an in-line arrangement.
To accomplish the first object of the present invention, there is provided an electron gun for a color cathode ray tube including means for generating three electron beams arranged in-line, means for forming an auxiliary lens for focusing and accelerating the electron beams generated by the electron beam generating means, and means for forming a main lens, for finally focusing and accelerating the electron beams focused and accelerated by the auxiliary lens forming means, and having first and second outer electrodes which face each other and in each of which a large electron beam aperture through which the three electron beams pass is formed, and first and second inner electrodes each installed inside the first and second outer electrodes and having three electron beam apertures shaped of vertically elongated squares.
In the present invention, recessed portions having different depths are formed at facing vertical peripheries of side electron beam apertures among the three electron beams of the first inner electrode, the recessed portions formed at the outer vertical peripheries being deeper than those formed at the vertical peripheries positioned near the central electron beam aperture. Also, recessed portions having the same depth are formed at facing vertical peripheries of side electron beam apertures among three electron beam apertures of the second inner electrode.